Devices and methods for separating particles

ABSTRACT

Embodiments of the present disclosure provide for devices, methods for separating particles, and the like.

CLAIM OF PRIORITY TO RELATED APPLICATION

This application is a continuation of co-pending U.S. patent applicationentitled “DEVICES AND METHODS FOR SEPARATING PARTICLES” having Ser. No.15/846,497, filed Dec. 19, 2017 which is divisional of co-pending U.S.patent application entitled “DEVICES AND METHODS FOR SEPARATINGPARTICLES” having Ser. No. 15/290,700, filed Oct. 11, 2016, now U.S.Pat. No. 9,873,126 which is a divisional of U.S. patent applicationentitled “DEVICES AND METHODS FOR SEPARATING PARTICLES” having Ser. No.13/873,424, filed on Apr. 30, 2013, now U.S. Pat. No. 9,517,474, whichclaims priority to U.S. provisional application entitled “DEVICES ANDMETHODS FOR SEPARATING PARTICLES” having Ser. No. 61/648,786, filed onMay 18, 2012, the contents of which are incorporated herein by referencein their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Agreement No. 5R21 GM104528-03, awarded by the National Institutes of Health. TheGovernment has certain rights in the invention.

BACKGROUND

Microfluidic particle and cell sorting plays an important role inenvironmental monitoring, disease diagnostics, and therapeutics. Sometechniques include labeling the particle or cell, however, thesetechniques have disadvantages. Thus, there is a need to developalternative techniques for particle sorting.

SUMMARY

In accordance with the purpose(s) of the present disclosure, as embodiedand broadly described herein, embodiments of the present disclosure, inone aspect, relate to a devices, methods for separating particles, andthe like.

In an embodiment, a device, among others, includes: a first fluid inletin a flow channel for flowing a first liquid including two or more typesof particles, wherein the first liquid includes a magnetic fluid and/oris mixed with the magnetic fluid; a magnetic device configured to directa non-uniform magnetic force onto the magnetic fluid and the particles;and a plurality of outlets, wherein the non-uniform magnetic forcecauses the types of particles to be separated and flow into differentoutlets.

In an embodiment, a method for separating particles, among others,includes: disposing at least two types of particles in a first fluid,wherein the first liquid includes and/or is mixed with the magneticfluid; flowing the magnetic fluid and the particles down a channel;exposing the magnetic fluid and the particles to a non-uniform magneticforce; and separating the types of particles.

Other structures, compositions, methods, features, and advantages willbe, or become, apparent to one with skill in the art upon examination ofthe following drawings and detailed description. It is intended that allsuch additional structures, systems, methods, features, and advantagesbe included within this description, be within the scope of the presentdisclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of this disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1A illustrates a schematic representation of the sorting devicewith permanent magnets and a microfluidic channel. FIG. 1B illustratesan image of protype device. Scale bar is 10 mm. FIG. 1C illustrates atopview of the device and relevant dimensions. The arrows indicatedirection of magnets' magnetization. FIG. 1D illustrates a cross-sectionof the device.

FIGS. 2A-2I illustrate an analytical three-dimensional simulation ofmagnetic field and force distributions in microfluidic channel, andtrajectories of cells. Simulation parameters match exact experimentalconditions. FIGS. 2A-2C show the x-y plane (z=0), FIGS. 2D-2F illustratethe y-z plane (x=0), FIGS. 2G-2I illustrate the x-z plane (y=0) ofmagnetic field strength (surface plot) (FIGS. 2A, 2D, 2G), magneticforce (surface plot: force magnitude; arrow plot: force direction)(FIGS. 2B, 2E, 2H), and particles' trajectories (FIGS. 2C, 2F, 2I). Dotsindicate starting points, while crosses indicate ending points of cells'trajectories. E. coli cell has volume range of 2.1-16.7 μm³ and Yeastcell has volume range of 180-382 μm³, resulting in a distribution oftrajectories for each type of cell. The triangle in FIG. 2C indicatesboundary between Outlets C and D. Dots indicate starting points, whilecrosses indicate ending points of cells' trajectories.

FIGS. 3A-3B illustrate cell viability test of Escherichia coli andSaccharomyces cerevisiae. FIG. 3A, top and bottom, are photos showingEscherichia coli and Yeast colonies formed in M9 medium and EMG 408ferrofluids after 10⁶ dilution from initial growth, respectively. FIG.3B illustrates Colony Forming Unites (CFU) count of Escherichia coli andSaccharomyces cerevisiae using initial growth cell concentration.

FIGS. 4A-4I illustrate experimental composite micrographs of sortingprocess. FIGS. 4A, 4D, 4G illustrate particles/cells mixture (FIG. 4A:Escherichia coli (green) and 7.3 μm particles (red); FIG. 4D:Saccharomyces cerevisiae (red and bright-field) and 1.0 μm particles(green); FIG. 4G: Escherichia coli (green) and Saccharomyces cerevisiae(red and bright-field) before magnetic fields were applied. FIGS. 4B,4E, 4H illustrate micrographs of Outlet C after magnetic fields wereapplied, and FIGS. 4C, 4F, 4I were micrographs of Outlet D. Bluetriangles indicate boundary between Outlets C and D. Scale barsrepresent 200 μm.

FIGS. 5A-5L illustrate experimental composite micrographs of sizedistribution analysis, including micrographs of particles/cells mixturecollected before sorting at Inlet A and after separation at Outlets Cand D, and remaining and separation efficiencies. FIGS. 5A-5D illustrateEscherichia coli and 7.3 μm particles mixture; FIGS. 5E-5H illustrateSaccharomyces cerevisiae and 1.0 μm particles mixture; FIGS. 5I-5Lillustrate Escherichia coli and Saccharomyces cerevisiae mixtures. Thebar with normal number on top shows remaining efficiency, while the barwith italic number on top shows separation efficiency. Scale barsrepresent 200 μm.

FIG. 6A illustrates a schematic representation of the sorting devicewith permanent magnets and a microfluidic channel. FIG. 6B illustratesan image of protype device. Scale bar is 10 mm. FIG. 6C illustrates atopview of the device and relevant dimensions. The arrows indicatedirection of magnets' magnetization. FIG. 6D illustrates a cross-sectionof the device.

FIG. 7 illustrates cell viability test of HeLa cell and red blood cellin Hank's Balanced Salt Solution (HBSS) and PEG ferrofluids, medium 1 isHBSS, medium 2 to 4 are PEG ferrofluid with magnetic nanoparticlesvolume fraction of 0.395%, 0.79%, 1% respectively. After 0, 1, and 2hours incubation, HeLa cell and red blood cell viability are countedwith trypan blue dye exclusion assay.

FIGS. 8A-8J illustrate experimental composite micrographs of focusingand sorting process. FIGS. 8A-8D illustrate HeLa cell and red bloodcells in channel before magnetic fields were applied. FIGS. 8E-8J weremicrographs after magnetic fields were applied. FIGS. 8H-8J illustratemicrographs of Outlet 1-6. Scale bars represent 200 μm.

FIG. 9 illustrates the separation efficiency verification of cellsdistribution in each outlet after separation.

FIG. 10A illustrates a schematic representation of the sorting devicewith permanent magnets and a microfluidic channel. FIG. 10B illustratesan image of protype device. Scale bar is 10 mm. FIG. 10C illustrates atopview of the device and relevant dimensions. The arrows indicatedirection of magnets' magnetization. FIG. 10D illustrates across-section of the device.

FIGS. 11A-11H illustrate an experimental composite micrographs ofsorting process. FIGS. 11A-11D were superimposed micrographs of 15.5 μmand 5.8 μm particles particles mixture. FIGS. 11E-11H illustrates HeLacell and 5.8 μm particles particles mixture. FIGS. 11A-11B, 11E, and 11Fillustrates micrographs of inlets and outlets before magnetic fieldswere applied. FIGS. 11C, 11D, 11G, 11H illustrate micrographs of inletsand outlets after magnetic fields were applied. Scale bars represent 200μm.

DISCUSSION

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present disclosure will be limited onlyby the appended claims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit (unlessthe context clearly dictates otherwise), between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the disclosure. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges and are also encompassed within the disclosure, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present disclosure, the preferredmethods and materials are now described.

All publications and patents cited in this specification are hereinincorporated by reference as if each individual publication or patentwere specifically and individually indicated to be incorporated byreference and are incorporated herein by reference to disclose anddescribe the methods and/or materials in connection with which thepublications are cited. The citation of any publication is for itsdisclosure prior to the filing date and should not be construed as anadmission that the present disclosure is not entitled to antedate suchpublication by virtue of prior disclosure. Further, the dates ofpublication provided could be different from the actual publicationdates that may need to be independently confirmed. Terms defined inreferences that are incorporated by reference do not alter definitionsof terms defined in the present disclosure or should such terms be usedto define terms in the present disclosure they should only be used in amanner that is inconsistent with the present disclosure.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentdisclosure. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of chemistry, material science, and the like,which are within the skill of the art. Such techniques are explainedfully in the literature.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toperform the methods and use the compositions and compounds disclosed andclaimed herein. Efforts have been made to ensure accuracy with respectto numbers (e.g., amounts, temperature, etc.), but some errors anddeviations should be accounted for. Unless indicated otherwise, partsare parts by weight, temperature is in ° C., and pressure is inatmosphere. Standard temperature and pressure are defined as 25° C. and1 atmosphere.

Before the embodiments of the present disclosure are described indetail, it is to be understood that, unless otherwise indicated, thepresent disclosure is not limited to particular materials, reagents,reaction materials, manufacturing processes, or the like, as such canvary. It is also to be understood that the terminology used herein isfor purposes of describing particular embodiments only, and is notintended to be limiting. It is also possible in the present disclosurethat steps can be executed in different sequence where this is logicallypossible.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a support” includes a plurality of supports. In thisspecification and in the claims that follow, reference will be made to anumber of terms that shall be defined to have the following meaningsunless a contrary intention is apparent.

Discussion

Embodiments of the present disclosure provide for devices, methods forseparating particles, and the like. An embodiment of the presentdisclosure is advantageous because it has a very high sorting efficiency(e.g., about 95% or more, about 99% or more, about 99.9% or more) and avery high throughput (e.g., about 10⁷ cells/hour or more, about 10⁸cells/hour or more). In addition, the device is less expensive thanother techniques (e.g., FACS) and is straightforward to operate.

In general, embodiments of the present disclosure include non-uniformmagnetic field-assisted processes and devices for the separation ofparticles (e.g., cells) within a magnetic fluid. Under non-uniformmagnetic fields, particles such as cells can experience the generatedmagnetic field direction to produce a magnetic buoyancy force, analogousto buoyancy force, as magnitude of the force is proportional to thevolume of cell. This force can be used to spatially separate cells ofdifferent sizes in certain flow conditions (e.g., laminar flow).Embodiments of the present disclosure can be label-free and/or do notrequire time-consuming steps of magnetic beads conjugation. Embodimentsof the present disclosure include high-efficiency and high-throughputcontinuous-flow particle separation and focusing devices using magneticfluid (e.g., ferrofluids) and magnets (e.g., permanent magnets).Permanent magnet based devices are low-cost and easy to operate andtheir operations does not generate heat. Magnetic fields produced bypermanent magnets are substantially larger than the ones bycurrent-carrying electrodes, which can increase the sorting throughputand efficiency of embodiments of the present disclosure.

In an exemplary embodiment, the device includes a first fluid inlet in aflow channel for flowing a first liquid including two or more types ofparticles, where the first liquid includes a magnetic fluid. In anembodiment, additional inlets can be present to introduce other reagentsor fluids. For example, the cells can be flowed in the first fluid inletand the magnetic fluid can be flowed in a second fluid inlet, where thetwo fluids mix in or prior to introduction to the channel. In anembodiment, the flow rate of the fluid(s) can be controlled and the flowrate can be used to enhance the separation.

In an embodiment the channel can have a constant diameter along itslength. In another embodiment, the channel can have a tapered diameter.In an embodiment, the flow chamber can be designed to optimize theseparation of the particles.

In an embodiment, the channel can have a length of about 5000 μm to20000 μm prior to splitting into two or more outlets (e.g., 2 to 100),and a diameter of about 1000 to 2000 μm. In an embodiment, the outlets(e.g., outlet channels) can have the same or different diameters and canindependently have a diameter of about 500 to 2000 μm. In an embodiment,the outlets can be designed (e.g., diameter, three-dimensionalorientation relative to the channel (e.g., offset from the axis of thechannel), and the like) to enhance the separation of the particles.

In an exemplary embodiment of the device, a magnetic device configuredto direct a non-uniform magnetic force onto particles is positioned at apoint of the channel. In an embodiment, the magnetic device can bepositioned relative to the split from the channel to the outlets. In anembodiment, the magnetic device is configured to direct the non-uniformmagnetic force onto particles from one side of the channel. As notedabove, the design of the device (e.g., the position of the magneticdevice and/or the outlets) can take into consideration the variouscomponents, the particles to be separated, and/or the magnetic fluid, toachieve the desired separation efficiency and/or throughput.

In an embodiment, the magnetic energy can be produced using a magneticdevice that includes one or more permanent magnets positioned to producea non-uniform magnetic field in at least a portion of the channel. In anembodiment, one permanent magnet is disposed on one side of the channelto generate the non-uniform magnetic force. The strength of the magneticfield can be selected based upon the configuration of the device, theparticles to be separated (e.g., the volume of the particles), and thelike. In another embodiment, the magnetic device includes three or moremagnets (e.g., 3, 4, 5, 6, 7, and so on) that can be used to form anon-uniform magnetic field within an area of the channel. The design,number of magnets used, the non-uniform magnetic field generated, andthe like, can be designed to separate particles.

As noted above, the device includes a plurality of outlets. Once thenon-uniform magnetic force acts upon the particles, the particles flowin the first fluid is altered so that certain types of particles flowinto one outlet and another type of particle flows into a differentoutlet.

In an embodiment where many different types of particles are to beseparated, then the outlets can be spaced apart along the length of thechannel and/or more than one magnet can be used along the length of thechannel in conjunction with the spacing of the outlet. Many differenttypes of configurations are envisioned that are consistent with theteachings of the present disclosure and are intended to be covered byclaims of this and future application.

In an embodiment, the particles can experience non-uniform magneticforce and are biologically compatible with the magnetic fluid. Inparticular, the particles can be separated by the magnetic buoyancyforce exerted upon them. In an embodiment, the particles can includecells, polystyrene microparticles, and a combination thereof. In anembodiment, the cells can include bacterial cells, yeast cells, bloodcells, cancer cells, neural cells, sperm cells, eggs, as well as typesof cells that have size difference can be distinguished by thistechnique. In an embodiment, the cells can include Escherichia coli,Saccharomyces cerevisiae, Lactobacillus casei, red blood cells, Jurkatcells, and HeLa cells. In an embodiment, the volume of the cells can beabout 5 to 3000 μm³.

In an embodiment, the particles are mixed with a magnetic fluid (e.g.,prior to introduction to the device and/or within the device). In anembodiment, the magnetic fluid is a colloidal mixture of nano-sizemagnetic particles (e.g., about 5 to 10 nm in diameter), covered by asurfactant, suspended in a compatible carrier medium. In an embodiment,the magnetic particles can be iron oxide particles, cobalt particles,cobalt ferrite particles, iron particles, and FePt particles, or acombination thereof, where the amount of the magnetic particles in themagnetic fluid can be about 1% (v/v) to 10% (v/v). In an embodiment, thesurfactant can include electric double layer surfactant, polymersurfactant, inorganic surfactant, or a combination thereof. In anembodiment, the carrier medium can include water, hydrocarbon oil,kerosene, or a combination there. In an embodiment, the magnetic fluidcan be a ferrofluid, paramagnetic solution, or a combination thereof.

As mentioned above, embodiments of the present disclosure include amethod for separating particles, where the device described herein canbe used to perform steps of the method. In an embodiment, the method isa continuous flow method. In an embodiment, the at least two types ofparticles are disposed in fluid including a magnetic fluid (e.g., afirst fluid). In an embodiment, the fluid including the particles andthe magnetic fluid are flowed down the channel. At a position in thechannel (e.g., a first area), the fluid is exposed to a non-uniformmagnetic force, where a magnetic device can be used to generate thenon-uniform magnetic force. In an embodiment, the particles experience amagnetic buoyancy force that causes the particles to separate from oneanother based on the volume of the particles. In an embodiment, theparticles can be separated from one another into two or more outlets.

In an embodiment, this process can be repeated for the particles thatare separated to increase efficiency and/or separate particles havingsimilar characteristics (e.g., volume). For example, the separated flowcan be recirculated through the same channel or can be flowed through adifferent channel is a device include two or more channels and magnets.

EXAMPLES

Now having described the embodiments of the disclosure, in general, theexample describes some additional embodiments. While embodiments of thepresent disclosure are described in connection with the example and thecorresponding text and figures, there is no intent to limit embodimentsof the disclosure to these descriptions. On the contrary, the intent isto cover all alternatives, modifications, and equivalents includedwithin the spirit and scope of embodiments of the present disclosure.

Example 1 Brief Introduction:

A new sorting scheme based on ferrofluid hydrodynamics(ferrohydrodynamics) was used to separate mixtures of particles and livecells simultaneously. Two species of cells, including Escherichia coliand Saccharomyces cerevisiae, as well as fluorescent polystyrenemicroparticles were studied for their sorting throughput and efficiencyusing a commercial ionic ferrofluid. To separate mammalian cellsincluding blood cells, cervical cancerous cells and epithelial cells, alab-customized ferrofluid was used. Ferrofluids are stable magneticnanoparticles suspensions. Under external magnetic fields, magneticbuoyancy forces exerted on particles and cells lead to size-dependentdeflections from their laminar flow paths and result in spatialseparation. We report the design, modeling, fabrication andcharacterization of the sorting devices. This scheme is simple, low-costand label-free compared to other existing techniques.

Introduction:

Microfluidic particle and cell sorting plays an important role inenvironmental monitoring (Liu et al. 2004; Beyor et al. 2008; Dharmasiriet al. 2010), disease diagnostics (Nagrath et al. 2007; Adams et al.2008; Hoshino et al. 2011), and therapeutics (Toner and Irimia 2005;Yung et al. 2009). Compared to high-specificity and label-based cellsorting techniques such as fluorescence-activated cell sorter (FACS)(Bonner et al. 1972) and magnetic-activated cell sorter (MACS) (Miltenyiet al. 1990), microfluidic sorting is mostly label-free, relying oncells' intrinsic properties such as size, shape, density, deformability,electric and magnetic properties for manipulation specificity (Pamme2007; Tsutsui and Ho 2009; Gossett et al. 2010; Lenshof and Laurell2010). When applicable, microfluidic sorting is favored over label-basedones, because they are inexpensive and require minimal user training foroperation (Gossett et al. 2010). Among them, those based on channeldesign including pinched flow fractionation (Yamada et al. 2004) anddeterministic lateral displacement (Huang et al. 2004; Davis et al.2006) combine laminar flows with mechanical structures to directparticles of different sizes into separate streamlines. Continuousinertial separation uses balance between inertial lift force and Deandrag force in curved channels for size-dependent sorting of particlesand cells (Di Carlo 2009). On the other hand, external energy inputssuch as acoustic, electric and magnetic forces have also been used tomanipulate cells in microfluidic systems. Depending on the application,their simpler channel geometry and faster manipulation speed mayoutweigh the complications of integrating electrodes in their designs.For example, acoustophoresis can separate particles and cells accordingto their size, density, as well as compressibility (Laurell et al. 2007;Shi et al. 2009; Wang and Zhe 2011). Dielectrophoresis (DEP), arisingfrom interactions of cells' dipoles and their surrounding electricfields, can realize low-cost and integrated devices for cellmanipulation (Voldman 2006). Magnetophoresis (MAP) takes advantages ofparamagnetic nature of red blood cells and magnetotactic bacteria andapplies non-uniform magnetic fields to separate them from non-magneticobjects (Zborowski et al. 2003; Lee et al. 2004). However, mostapplications of magnetophoresis use functionalized magnetic beads forlabeling (Pamme 2006; Liu et al. 2009; Gijs et al. 2010). Thelabel-based methods are manually intensive and time-consuming. Themagnetic moments of these beads, even from the same batch, can varydramatically due to their manufacturing procedure, making scaling of themethod scaling difficult (Hafeli et al. 1997; Miller et al. 2001; Rifeet al. 2003; Mihajlovic et al. 2007; Shevkoplyas et al. 2007).

To address problems with label-based magnetophoresis, a label-freetechnique that uses reverse magnetophoresis to manipulate and sort cellshas been developed recently based on ferrofluid hydrodynamics(ferrohydrodynamics) (Yellen et al. 2005; Kose et al. 2009; Zhu et al.2010; Zhu et al. 2011a; Kose and Koser 2012). Ferrofluids are colloidalsuspensions of magnetic nanoparticles, typically magnetite (Fe₃O₄) withapproximately 10 nm diameters (Rosensweig 1985). They are covered byeither electrostatic or steric surfactants to keep them fromagglomeration due to van der Waals force and in suspension within awater or oil medium. Ferrohydrodynamics studies mechanics of ferrofluidmotion under external magnetic fields (Rosensweig 1985; Odenbach andEditor 2002). Its applications in microfluidics, recently reviewed byNguyen (Nguyen 2012), include miniaturized polymerase chain reaction(PCR) (Sun et al. 2007; Sun et al. 2008), traveling-wave magnetic fieldpumping (Mao and Koser 2006; Mao et al. 2011), micro-scale mixing (Maoand Koser 2007), micropump (Hatch et al. 2001; Love et al. 2004), anddroplet manipulation (Nguyen et al. 2006; Zhang et al. 2011b, a).

In applications of cell manipulation, the purpose of using ferrofluidsis to induce effective magnetic dipole moments within cells. Undernon-uniform magnetic fields, cells will experience in the weaker fielddirection a magnetic buoyancy force, analogous to buoyancy force, asmagnitude of the force is proportional to the volume of cell (Rosensweig1985). Many groups have been working on adapting this principle toparticles and cells sorting. For example, Whitesides' group separatedsynthetic particles according to their densities' difference usingparamagnetic salt solutions (Winkleman et al. 2007; Mirica et al. 2009).Pamme's group demonstrated continuous particle and cell manipulationusing paramagnetic salt solution in microfluidic devices (Peyman et al.2009; Rodriguez-Villarreal et al. 2011). Xuan's group studied thetransport of particles in both paramagnetic solutions and ferrofluidsthrough a rectangular microchannel embedded with permanent magnets(Liang et al. 2011; Zhu et al. 2012). Park's group recently sorted humanhistolytic lymphoma monocytes cells from red blood cells usinggadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) solution (Shenet al. 2012). However, magnetic susceptibility of paramagnetic saltsolutions is inherently small, about 5 orders of magnitude weaker thanthat of a ferrofluid (Krebs Melissa et al. 2009), rendering slowermanipulation speed and low throughput. As a result of the highersusceptibility of ferrofluids, Koser's group was able to use anintegrated microfluidic platform for sorting of microparticles and livecells within a citrate stabilized cobalt-ferrite ferrofluid in staticflow conditions (Kose et al. 2009). The same device was also applied tocontinuous-flow frequency-adjustable particles separation (Kose andKoser 2012). Our group developed high-efficiency and high-throughputcontinuous-flow particle separation and focusing devices usingcommercial ferrofluids and hand-held permanent magnets (Zhu et al. 2010;Zhu et al. 2011b; Zhu et al. 2011a). Permanent magnet based devices arelow-cost and easy to operate; their operations do not generate heat.Magnetic fields produced by permanent magnets are substantially largerthan the ones by current-carrying electrodes.

High throughput, label-free and selective cell sorting realized in asingle automated device can have profound impacts on environmentalmonitoring, diagnostics and therapeutics. Although continuous-flowferrohydrodynamic sorting has been demonstrated with microparticles, ithas not previously been reported with live cells (Zhu et al. 2010). Thepotential for live cell applications of continuous-flowferrohydrodynamic sorting motivates the study presented here. Wedeveloped a microfluidic device that could continuously sort cells ofdifferent sizes based on ferrohydrodynamics, which involved manipulationof cells within ferrofluids via external non-uniform magnetic fields.When cell mixtures and ferrofluids were injected into the channel by apressure-driven flow, deflections of cells from their laminar flow pathswould occur because of the magnetic field gradient and resultingmagnetic buoyance force. This deflection will lead to spatial separationof cells of different sizes at the end of channel.

In the following sections, we first summarize materials and methods usedin this study, followed by results from a three-dimensional theoreticalstudy of cells' transport in the microfluidic device. In the firstembodiment, cell viabilities of Escherichia coli and Saccharomycescerevisiae in a commercial ferrofluid are discussed. Afterwards,calibration of the sorting device with fluorescent polystyrenemicroparticles is performed. Escherichia coli and Saccharomycescerevisiae are sorted in the device, and cells distribution is analyzedon samples collected from channel outlets. In the second embodiment,cell viabilities of red blood cells, HeLa cells, in the customizedferrofluid are tested. A stronger magnetic field gradient is applied toenable particles focusing. Calibration of the sorting device withpolystyrene microparticles of comparable size is performed beforesorting the cells. Cell distribution is analyzed on samples collectedfrom six channel outlets. In the third embodiment, a new one-stepseparation micro-device without the need of washing cells afterenrichment was developed. This device takes in cells sample in itsnatural reagent or ferrofluids, separate them based on their sizes, andreturn purified cells to an outlet. It eliminates the preparation andwashing steps typically associated with our device. In the end we willdiscuss outlook of ferrohydrodynamic sorting.

Materials and Methods:

The prototype polydimethylsiloxane (PDMS) microfluidic device wasfabricated through a standard soft-lithography approach and attached toa flat surface of another piece of PDMS, as shown in FIGS. 1A-1B. A maskof the device pattern was created using AutoCAD 2008 (Autodesk Inc., SanRafael, Calif.) and printed by a commercial photo-plotting company(CAD/Art Services Inc, Bandon, Oreg.). In the first embodiment,dimensions of the microfluidic channel are listed in FIGS. 1C-1D.Thickness of the device was measured to be 38 μm by a profilometer(Dektak 150, Veeco Instruments Inc., Chadds Ford, Pa.). Beforeattachment, PDMS surfaces were treated with plasma (PDC-32G plasmacleaner, Harrick Plasma, Ithaca, N.Y.) at 11.2 Pa O₂ partial pressurewith 18 W power for 1 minute. A stack of four NdFeB permanent magnetswas embedded into PDMS channel with their magnetization directionvertical to the channel during curing stage. Each magnet is 5 mm inwidth, 5 mm in length and 2 mm in thickness. The magnet stack was placed2 mm away from the channel. Flux density at the center of magnetsstack's surface was measured to be 470 mT by a Gauss meter (Model 5080,Sypris, Orlando, Fla.) and an axial probe with 0.381 mm diameter ofcircular active area. In the second embodiment, dimensions of themicrofluidic channel are listed in FIGS. 6C-6D. Thickness of the devicewas measured to be 50 μm by a profilometer (Dektak 150, VeecoInstruments Inc., Chadds Ford, Pa.). A NdFeB permanent magnets wasembedded into PDMS channel with their magnetization direction verticalto the channel during curing stage. The magnet is 2.54 cm in width, 6.35mm in length and thickness. The magnet stack was placed 1 mm away fromthe channel. Flux density at the center of magnets stack's surface wasmeasured to be 470 mT. In the third embodiment, dimensions of themicrofluidic channel are listed in FIGS. 10C-10D. Thickness of thedevice was measured to be 50 μm by a profilometer (Dektak 150, VeecoInstruments Inc., Chadds Ford, Pa.). A NdFeB permanent magnets wasembedded into PDMS channel with their magnetization direction verticalto the channel during curing stage. The magnet is 2.54 cm in width,3.175 mm in length and thickness. The magnet stack was placed 2 mm awayfrom the channel. Flux density at the center of magnets stack's surfacewas measured to be 470 mT. Before liquid injection, the device wastreated with plasma for 10 minutes to render PDMS surfaces hydrophilic.This step ensured both cells and microparticles would not attach ontoPDMS surfaces during sorting.

In the first embodiment, we used a commercial water-based, pH˜7magnetite ferrofluid coated with anionic surfactants (EMG 408, FerrotecCo., NH). Volume fraction of magnetite particles in this ferrofluid is1.1%. Mean diameter of nanoparticles has been determined fromTransmission Electron Microscopy (TEM) images to be ˜10 nm. Initialmagnetic susceptibility was measured to be 0.26; saturationmagnetization was 60 Gauss; viscosity was 1.2×10⁻³ kg/m·s. Escherichiacoli (strain MG1655) and Saccharomyces cerevisiae (Baker's yeast), andtwo fluorescent microparticles (green 1.0 μm diameter, Thermo FisherScientific Inc., Waltham, Mass., and red 7.3 μm diameter, BangsLaboratories Inc., Fishers, Ind.) were used in sorting. In the secondand third embodiment, we used a water-based, pH˜7 maghemite ferrofluidcoated with polyethylene glycol copoplymer (ATLOX4913, Croda. Inc).Volume fraction of magnetite particles in this ferrofluid is 1%. Meandiameter of nanoparticles has been determined from Transmission ElectronMicroscopy (TEM) images to be ˜5.13 nm. Initial magnetic susceptibilitywas measured to be 0.098; saturation magnetization was 2.93 kA/m.

Ferrofluid and particles/cells mixture injected into microchannel weremaintained at tunable flow rates using a syringe pump (Nexus 3000,Chemyx Inc., Stafford, Tex.). Sorting was conducted on the stage of aninverted microscope (Zeiss Axio Observer, Carl Zeiss Inc., Germany).Micrographs of cells and particles were recorded through either a greenfluorescent filter set (41001 FITC, Chroma Technology Corp., Rockingham,Vt.), or a red filter set (43HE, Carl Zeiss Inc., Germany), and a CCDcamera (SPOT RT3, Diagnostic Instruments, Inc., Sterling Heights,Mich.). Cell samples collected from channel outlets were pipetted ontomicroscope slides and analyzed using a high-resolution CCD camera(AxioCam HR, Carl Zeiss Inc., Germany) for size distributions toquantitatively evaluate efficiency of this approach. ImageJ® softwarewas used to count the number of cells.

Saccharomyces cerevisiae (Baker's yeast) cells were first grown in a 10ml test tube containing 2 ml of YPG medium (10 g/l yeast extract, 20 g/lglucose, 20 g/l glucose) overnight. They were then transferred into a100 ml shake flask containing 20 ml of YPG medium. After 4 h growth at30° C. and 250 rpm, cells in the flask were stained with fluorophores.Escherichia coli (strain MG1655) cells were first grown in a 10 ml testtube containing 2 ml of Luria-Bertani (LB) medium overnight. They werethen transferred into a 100 ml shake flask containing 20 ml of LB medium(25 g/l LB). After 4 h growth at 37° C. and 250 rpm, cells were stainedwith fluorophores. Nucleic acid stains SYTO9 (green) and SYTO17 (red)(Molecular Probes Inc., Eugene, Oreg.) were used in cell staining.

Hela cells were cultured in culture flasks (BD Falcon) containing 12 mLof DMEM medium with 10% (v/v) fetal bovine serum, 100 U/mL penicillin,and 100 μg/mL streptomycin. All cell lines were incubated (5% CO₂, 90%humidified) at 37° C. in an incubator (Innova-Co 170; New BrunswickScientific, U.K.) prior to use. Cells were subcultured at a ratio of 1:5every 3 days to maintain cells in the exponential growth phase. Cellswere detached from the flask with the treatment of 0.25% (w/v)trypsin-EDTA solution (Gibco) for 3 min for harvest. Cells were thensuspended in the culture media at a concentration of 2×10⁶ cells/mLbefore use.

To study of viability of Escherichia coli and Saccharomyces cerevisiaecells exposed to EMG 408 ferrofluids, nominally 2×10⁹ cells Escherichiacoli and 2×10⁷ cells Saccharomyces cerevisiae grown as described abovewere centrifuged twice at 4° C. and washed in defined M9 medium (6.78g/l Na₂HPO₄, 3.0 g/l KH₂PO₄, 0.5 g/l NaCl, 1.0 g/l NH₄Cl) without carbonsource. For either cell type in duplicate, the washed cell pellet fromcentrifugation was combined with either 2 ml of EMG 408 ferrofluid or 2ml M9 medium as a control. After 2 hours of incubation at roomtemperature in these fluids, cell density was determined in triplicateusing standard microbial serial dilutions (10⁶ dilution for Escherichiacoli, and 10⁴ dilution for Saccharomyces cerevisiae), with thetransferring of known volumes to Petri plates and counting of ColonyForming Units (CFU) after 24 hours.

To study the viability of HeLa, red blood cells exposed toPEG-ferrofluids, nominally 2×10⁶ cells grown as described above werecentrifuged twice at 4° C. and washed in Hank's buffer solution (HBSS).For either cell type in duplicate, the washed cell pellet fromcentrifugation was combined with either 1 ml of PEG ferrofluid or 1 mlHBSS as a control. After 2 hours of incubation at room temperature inthese fluids, cell viability was determined with trypan blue stainingand counted with a haemocytometer.

Theory and Simulation:

Previously, we reported both two-dimensional (2D) and three-dimensional(3D) analytical models for microfluidic transports of microparticles inferrofluids (Zhu et al. 2011a; Zhu et al. 2011b). In this work, weapplied the 3D analytical model to predict cells' sorting in permanentmagnet based device. Briefly, we obtained cells' trajectories by firstcalculating magnetic buoyancy force on cells using a 3D analytical modelof magnetic fields (Furlani and Sahoo 2006) and a nonlinearmagnetization model of ferrofluids (Rosensweig 1985), and then solvinggoverning equations of motion for cells in laminar flow condition (Brodyet al. 1996). All relevant parameters used in our simulation are listedin FIGS. 1A-1D and Materials and Methods section. In addition, wecalculated volume of a single rod-shape Escherichia coli cell with shortaxis of 0.5-1 μm and long axis of 2-4 μm to be 2.1-16.7 μm³ (Kaya andKoser 2009), and volume of a single sphere-shape Saccharomycescerevisiae cell with diameter of 7-9 μm to be 180-382 μm³ (Jorgensen etal. 2002).

FIGS. 2A-2I summarize simulated distribution of magnetic fields andmagnetic buoyance forces in the sorting channel, as well as 3Dtrajectories of Escherichia coli and Saccharomyces cerevisiae cells. Thesurface plot in FIG. 2A shows magnitude of magnetic fields of x-y planeat z=0. Magnetic fields decayed rather quickly from the surface of themagnet and formed a gradient that resulted in magnetic buoyance force oncells in both x and y directions, as indicated in FIG. 2B. Consequently,cells experiencing such force when entering the sorting channel woulddecelerate in x direction and accelerate my direction. Force computed ona spherical microparticle of 7.3 μm diameter, with its total volume(˜200 μm³) close to that of a single Saccharomyces cerevisiae cell, ison the order of 10 pN. Cell mixtures were quickly sorted by magneticbuoyancy force towards the end of channel, as shown in FIG. 2C withsimulated cells' trajectories considering their natural size variations.All Escherichia coli cells, having much smaller size and volume comparedto Saccharomyces cerevisiae cells, exited the channel through Outlet D,while all Saccharomyces cerevisiae cells migrated towards Outlet C.FIGS. 2D-2F illustrate distribution of magnetic fields and forces, aswell as trajectories of cells of y-z plane at x=0; FIGS. 2G-2I depictthe cases of x-z plane at y=0. We are interested in 3D trajectories ofcells, in part due to the opaqueness of ferrofluids and difficulty inrecording cells' weak fluorescence in the channel, especially the redfluorescent from Saccharomyces cerevisiae cells, as shown later in theresults. In a concentrated ferrofluid (˜10% v/v), particles and cellsare visible only when they are very close (˜1 μm) to the surface ofchannel (Zhu et al. 2011b). Visibility was a less of a problem whendiluted ferrofluids (˜1% v/v) and thin microchannel were used in ourdevice. Simulation results from FIG. 2F and FIG. 21 indicated in ourcurrent setup all cells were pushed towards the channel bottom surface,which would enhance visibility of stained cells.

Results and Discussions: Cell Viability

FIG. 3A shows the CFU in both M9 medium and EMG 408 ferrofluids afterincubation. Counts of CFU for each case were averaged over 3 plates andplotted in FIG. 3B. We observed a slight increase in cell density after2 hours of incubation in the ferrofluid compared to the M9 mediumcontrol for both cell types, suggesting a possibility that either theEMG 408 ferrofluid acted as a cell protectant or the cells continued togrow in this ferrofluid during incubation. Nonetheless, this ferrofluidwas not detrimental to the viability of both cell types after 2 hours ofexposure, which allowed enough time to carry out the sorting procedure.

FIG. 7 shows the viability of HeLa cells and mouse red blood cells inboth HBSS and PEG ferrofluids after incubation. We counted the cellnumbers with trypan blue viability staining after 2 hours of incubationin the ferrofluid compared with the HBSS control for both cell types,

Cells Sorting:

In the first embodiment, we first calibrated the sorting device using amixture of Escherichia coli cells and red fluorescent 7.3 μm particles,which have similar total volume of Saccharomyces cerevisiae cells.Washed Escherichia coli cell pellet from centrifugation as describedabove was stained with 1 μl of green nucleic acid stain SYTO9. Bothparticles and cells have concentrations of ˜10⁷ counts/ml. We introducedmicroparticles/cells mixture into microfluidic channel Inlet A at aconstant flow rate of 1.5 μl/min. The mixture was hydrodynamicallyfocused into a narrow stream by sheath flow from Inlet B at a flow rateof 6 μl/min. The observation window was located right before the channeloutlets, as indicated in FIG. 1C. When magnetic fields were off,particles and cells were observed in fluorescent mode flowing togethernear sidewall of the channel and exiting through Outlet D, as shown incomposite micrograph of FIG. 4A. When magnetic fields were on, magneticbuoyancy forces deflected particles from their laminar flow pathstowards Outlet C, as shown in FIG. 2B. On the other hand, forces onsmaller Escherichia coli cells were inadequate to deflect them to OutletC; therefore they exited the channel through Outlet D still, as shown inFIG. 2C. This resulted in spatial separation of particles/cells mixtureat the end of channel. We were able to separate ˜10⁶ particles from ˜10⁶cells per hour with 1.5 μl/min flow rate. Simply increasing the flowrate can further increase sorting throughput. Current microfluidicsorting schemes use flow rates ranging between ˜10 μl/min and ˜1 ml/min(Gossett et al. 2010). With such flow rates and 10⁷-10⁸ cells/mlconcentration, maximum sorting throughput of our device in theory can goup to 10⁹ cells per hour.

Secondly, we calibrated the device using a mixture of Saccharomycescerevisiae cells and green fluorescent 1.0 μm particles, which havesimilar volume as Escherichia coli cells. Saccharomyces cerevisiae werestained with red nucleic acid stain SYTO17. Both particles and cellsagain have concentrations of ˜10⁷ counts/ml. Due to weak redfluorescence from SYTO17 in our setup, we chose to use a combination ofbright-field and fluorescent modes microscopy to record the sortingprocess. FIG. 4D shows merged composite micrograph of green fluorescent1.0 μm particles and bright-field particles/Saccharomyces cerevisiaemixture, both of which exited channel through Outlet D when magneticfields were off. Sorting of this mixture was achieved as soon asmagnetic fields were on, as depicted in FIGS. 4E-4F. Cells distributionanalysis presented in the following section confirmed a close to 100%sorting efficiency. Sorting throughput was ˜10⁶ cells per hour. Here wedemonstrated that combination of bright-field and fluorescent microscopycan successfully circumvent recording issues originating from opaquenessof ferrofluids and weak fluorescence from stained live cells.

Finally, sorting of Escherichia coli and Saccharomyces cerevisiae cellswere carried out in the same device at the same time. Escherichia colicells were stained with green fluorescence while Saccharomycescerevisiae were stained with red fluorescence. Both types of cells wereadjusted to ˜10⁷ cells/ml concentration in initial mixture. It isclearly shown in FIG. 4G that all cells exited from the channel throughOutlet D when there was no magnetic field. Both bright-field andfluorescent mode micrographs of cells were recorded and merged to formFIG. 4G. Saccharomyces cerevisiae cells were successfully sorted fromthe initial cell mixture with the application of magnetic fields, asdemonstrated in FIGS. 4H-4I.

In the second embodiment, mouse red blood cell pellet were collectedfrom centrifugation of whole blood. Both red blood cells and Hela cellshave concentrations of ˜2×10⁶ counts/ml. We introduced cells mixtureinto microfluidic channel Inlet A at a constant flow rate of 8 μl/min.The mixture was hydrodynamically focused into a narrow stream by sheathflow from Inlet B at a flow rate of 14 μl/min. The observation windowwas located at four different sections, as indicated in FIGS. 8A-8D.When magnetic fields were off, particles and cells were flowing togethernear sidewall of the channel and exiting through Outlet 1, as shown incomposite micrograph of FIG. 8D. When magnetic fields were on, magneticbuoyancy forces deflected particles towards the inlet wall as shown inFIG. 8E, then hydrodynamically focused into a narrow stream as shown inFIG. 8F. HeLa Cells were separated due to the larger size as shown inFIG. 8G and deflected towards outlet 5-6, as shown in FIG. 8J. On theother hand, forces on smaller red blood cells were inadequate; thereforethey exited the channel through Outlet 1-4, as shown in Figures FIGS.8H-8I. This resulted in spatial separation of cells mixture at the endof channel.

FIGS. 11A-11D were superimposed micrographs of 15.5 μm and 5.8 μmparticles particles mixture. FIGS. 11E-11H illustrate HeLa cell and 5.8μm particles particles mixture. FIG. 11A, FIG. 11B, FIG. 11E, and FIG.11F illustrate micrographs of inlets and outlets before magnetic fieldswere applied. FIG. 11C, FIG. 11D, FIG. 11G, and FIG. 11H illustratemicrographs of inlets and outlets after magnetic fields were applied.

In the third embodiment, we demonstrated sorting process using a mixtureof 5.8 μm and 15.5 μm polystyrene particles. Both particles and cellshave concentrations of ˜2×10⁶ counts/ml. We introducedmicroparticles/cells mixture in HBSS or ferrofluids into microfluidicchannel Inlet A at a constant flow rate of 1.5 μl/min. The mixture washydrodynamically focused into a narrow stream by sheath flow from InletB at a flow rate of 4 μl/min and a third HBSS flow at a flow rate of 6μl/min. The observation window was located right before the channeloutlets, as indicated in FIG. 10C. When magnetic fields were off,particles and cells were flowing together near sidewall of the channeland exiting through Outlet 1, as shown in composite micrograph of FIG.11B. When magnetic fields were on, magnetic buoyancy forces deflectedparticles from their laminar flow paths towards Outlet 3, as shown inFIG. 11D. On the other hand, forces on smaller Escherichia coli cellswere inadequate to deflect them to Outlet 3; therefore they exited thechannel through Outlet 1 and 2, as shown in FIG. 11D. This resulted inspatial separation of particles/cells mixture at the end of channel.Simply increasing the flow rate can further increase sorting throughput.Current microfluidic sorting schemes use flow rates ranging between ˜10μl/min and ˜1 ml/min (Gossett et al. 2010). With such flow rates and10⁷-10⁸ cells/ml concentration, maximum sorting throughput of our devicein theory can go up to 10⁹ cells per hour.

Cell Sorting Efficiency:

In order to precisely evaluate sorting efficiency, in the firstembodiment, we collected samples from both Outlets C and D and analyzedthem for size distributions off chip. We stained cells in distinctivefluorescence and counted them using ImageJ® software. Specifically, infirst calibration, Escherichia coli cells were green and 7.3 μmparticles were red; in second calibration, Saccharomyces cerevisiaecells were red and 1.0 μm particles were green; in cells sorting,Saccharomyces cerevisiae cells were red and Escherichia coli cells weregreen. Fluorescent mode was chosen for distribution analysis to avoidmiscounting of cell types in bright-field micrographs. A magnetic fieldwas applied to push all particles and cells onto a surface of glassslide to increase visibility. We define remaining efficiency as ratio ofnumber of particles or cells exiting from Outlet D after magnetic fieldapplication to their initial number before magnetic field application.Similarly, sorting efficiency is defined as the ratio of number ofparticles or cells exiting from Outlet C after magnetic fieldapplication to their initial number before magnetic field application.FIG. 5A shows a representative composite micrograph of Escherichia colicells and 7.3 μm particles collected from Inlet A before sorting. 100%of 7.3 μm particles migrated to Outlet C and 98.8% Escherichia colicells remained in Outlet D, as depicted in FIGS. 5B-5C. Remaining andseparation efficiencies for both particles are plotted in FIG. 5D. FIGS.5E-5H and FIGS. 5I-5L show micrographs and efficiencies forSaccharomyces cerevisiae cells/1.0 μm particles mixture sorting andSaccharomyces cerevisiae cells/Escherichia coli cells mixture sorting,respectively. Both cases have 100% efficiencies. It should be noted thatsamples collected from Outlets C and D were greatly diluted byferrofluid sheath flow from Inlet B, rendering much lower particles andcells concentration for distribution analysis. A possible solution tothis problem is integration of cell focusing (Zhu et al. 2011a) andsorting steps on one chip.

In the second embodiment, we collected samples from multiple outlets andanalyzed them for size distributions with a haemocytometer. Phasecontrast microscopy was used to visualize cells in bright-fieldmicrographs. FIG. 9 shows the cells distribution in each outlet. 100% ofHeLa cells migrated to Outlet 5 and 6 while 100% mouse red blood cellsmigrated to Outlet 1-4.

In the third embodiment, 100% of 15.5 μm particles migrated to Outlet 3and 100% 5.8 μm particles migrated to Outlet 1 and 2, as counted with ahaemocytometer.

Outlooks of Ferrohydrodynamic Sorting:

Ferrohydrodynamic cell sorting offers the potential for high throughput(˜10⁷ cells/hour in this study and ˜10⁹ cells/hour in theory) and highseparation efficiency (˜100%) that are comparable to existingmicrofluidic sorting techniques but without the use of labels. Theassociated device is inexpensive and simple, only requiring a channeland hand-held permanent magnets. Sorting specificity of this approach isnot limited to size difference only; it is also sensitive to cells'shape and deformability (Kose et al. 2009). In adapting it tominiaturized flow cytometry, ferrohydrodynamic manipulation can firstfocus cells into single cell streams before sorting, eliminating needsfor excessive sheath flow and preventing sample dilution (Zhu et al.2011a). Compared to paramagnetic solution based sorting, ferrofluidoffers much higher magnetic susceptibility, eliminating needs for eithermicrofabricated ferromagnetic structures to enhance field gradient orhypertonic concentrations of paramagnetic salts that are notbiocompatible for live cell manipulation.

On the other hand, using water-based ferrofluids for cell manipulationis a work in progress. Diagnostic and research applications directedtowards simply purifying or isolating cells of interest from complexmixtures such as blood and exfoliated cytology specimens are exciting.For instance, blood cells obscure the detection of the larger but rareabnormal cervical cells from Pap test specimens and metastaticepithelial tumor cells circulating in blood (Moriarty et al. 2009; Yu etal. 2011). Misinterpreted cervical cytology ranks third among causes ofmedical negligence claims against pathologist (Frable 2007). A simple,low-cost tumor cell enrichment platform would benefit cancer screening.However, two issues, cell visibility and biocompatibility of mammaliancells in ferrofluids, limit applications of ferrohydrodynamicmanipulation. Ferrofluids are opaque due to light diffraction from theirhigh concentration of magnetic nanoparticles. Fluorescent cells need tobe close to channel surface for microscopic recording. In order toaddress this issue, ferrofluids with low solid content, as well asshallow microfluidic channel, are favored for cell manipulation. Inaddition, magnetic fields can be used to push cells onto channelsurface, increasing visibility of cells in fluorescent mode. In thisstudy, we used a combination of both bright-field and fluorescent modesmicroscopy to circumvent the opaqueness issue. Cells were readilyvisible in a shallow channel in bright-field micrographs. Anotherpotential issue is biocompatibility of ferrofluids. Our next step is toextend this methodology to mammalian cells, particularly human specimenssuch as blood and other bodily fluids, exfoliated musical cells, andtumor aspirates. The requirements of mammalian cells may differ fromEscherichia coli and Saccharomyces cerevisiae. For cell manipulation,materials, pH value, and surfactants of ferrofluids need to be renderedbiocompatible, at the same time the overall colloidal system offerrofluids must be maintained. Typically, nanoparticles withinferrofluids for cell applications are made of magnetite (Pankhurst etal. 2003). pH value of ferrofluids needs to be compatible with cellculture and maintained at 7.4. Salt concentration, tonicity, andsurfactant must be carefully chosen close to physiological conditions toreduce cell death. Although these are stringent requirements, progresshas been made towards synthesizing biocompatible ferrofluids. Forexample, Koser's group used citrate to stabilize cobalt-ferritenanoparticles for live red blood cell and Escherichia coli cell sorting(Kose et al. 2009). Yellen's group used Bovine Serum Albumin (BSA) tostabilize magnetite nanoparticles for human umbilical vein endothelialcells manipulation (Krebs Melissa et al. 2009). Viability tests fromboth studies have shown cells were able to retain their viability for upto several hours in ferrofluids. In our study, a commercially availablepH˜7 magnetite ferrofluid was able to sustain viability of bothEscherichia coli and Saccharomyces cerevisiae cells for at least 2hours.

Conclusion

In conclusion, we have developed a label-free and continuous-flowferrohydrodynamic cell sorting device and applied it in separatingEscherichia coli and Saccharomyces cerevisiae cells. A commercialmagnetite ferrofluid was used to separate particle and cell mixtures. Alab customized ferrofluid was used to separate mammalian cells.Construction of our device is simple and low-cost; we choose to usepermanent magnets instead of integrated electrodes to eliminate complexmicrofabrication process and auxiliary power supply. Current sortingthroughput is 10⁷ cells/hour, and sorting efficiency is close to 100%.We envision this device can achieve up to two orders higher throughputwhile still maintaining current sorting efficiency.

REFERENCES, EACH OF WHICH IS INCORPORATED HEREIN BY REFERENCE

-   Adams A A, Okagbare P I, Feng J, Hupert M L, Patterson D, Gottert J,    McCarley R L, Nikitopoulos D, Murphy M C, Soper S A (2008) Highly    efficient circulating tumor cell isolation from whole blood and    label-free enumeration using polymer-based microfluidics with an    integrated conductivity sensor. J Am Chem Soc 130 (27):8633-8641.    doi:Doi 10.1021/Ja8015022-   Beyor N, Seo T S, Liu P, Mathies R A (2008) Immunomagnetic    bead-based cell concentration microdevice for dilute pathogen    detection. Biomed Microdevices 10 (6):909-917. doi:Doi    10.1007/S10544-008-9206-3-   Bonner W A, Sweet R G, Hulett H R, Herzenbe. La (1972) Fluorescence    Activated Cell Sorting. Review of Scientific Instruments 43    (3):404-409.-   Brody J P, Yager P, Goldstein R E, Austin R H (1996) Biotechnology    at low Reynolds numbers. Biophys J 71 (6):3430-3441.-   Davis J A, Inglis D W, Morton K J, Lawrence D A, Huang L R, Chou S    Y, Sturm J C, Austin R H (2006) Deterministic hydrodynamics: Taking    blood apart. P Natl Acad Sci USA 103 (40):14779-14784. doi:Doi    10.1073/Pnas.0605967103-   Dharmasiri U, Witek M A, Adams A A, Osiri J K, Hupert M L, Bianchi T    S, Roelke D L, Soper S A (2010) Enrichment and Detection of    Escherichia coli O157:H7 from Water Samples Using an Antibody    Modified Microfluidic Chip. Analytical Chemistry 82 (7):2844-2849.    doi:Doi 10.1021/Ac100323k-   Di Carlo D (2009) Inertial microfluidics. Lab on a Chip 9    (21):3038-3046. doi:Doi 10.1039/B912547g-   Frable W J (2007) Error reduction and risk management in    cytopathology. Semin Diagn Pathol 24 (2):77-88.-   Furlani E P, Sahoo Y (2006) Analytical model for the magnetic field    and force in a magnetophoretic microsystem. J Phys D Appl Phys 39    (9):1724-1732. doi:Doi 10.1088/0022-3727/39/9/003-   Gijs M A M, Lacharme F, Lehmann U (2010) Microfluidic Applications    of Magnetic Particles for Biological Analysis and Catalysis. Chem    Rev 110 (3):1518-1563.-   Gossett D R, Weaver W M, Mach A J, Hur S C, Tse H T K, Lee W, Amini    H, Di Carlo D (2010) Label-free cell separation and sorting in    microfluidic systems. Analytical and Bioanalytical Chemistry 397    (8):3249-3267.-   Hafeli U, Schutt W, Teller J, Zborowski M (1997) Scientific and    clinical applications of magnetic carriers. Springer, New York.-   Hatch A, Kamholz A E, Holman G, Yager P, Bohringer K F (2001) A    ferrofluidic magnetic micropump. Microelectromechanical Systems,    Journal of 10 (2):215-221.-   Hoshino K, Huang Y Y, Lane N, Huebschman M, Uhr J W, Frenkel E P,    Zhang X J (2011) Microchip-based immunomagnetic detection of    circulating tumor cells. Lab on a Chip 11 (20):3449-3457. doi:Doi    10.1039/C11c20270g-   Huang L R, Cox E C, Austin R H, Sturm J C (2004) Continuous particle    separation through deterministic lateral displacement. Science 304    (5673):987-990. doi:10.1126/science.1094567 304/5673/987-   Jorgensen P, Nishikawa J L, Breitkreutz B J, Tyers M (2002)    Systematic identification of pathways that couple cell growth and    division in yeast. Science 297 (5580):395-400. doi:Doi    10.1126/Science.1070850-   Kaya T, Koser H (2009) Characterization of Hydrodynamic Surface    Interactions of Escherichia coli Cell Bodies in Shear Flow. Physical    Review Letters 103 (13):138103. doi:Artn 138103 Doi    10.1103/Physrevlett.103.138103-   Kose A R, Fischer B, Mao L, Koser H (2009) Label-free cellular    manipulation and sorting via biocompatible ferrofluids. P Natl Acad    Sci USA 106 (51):21478-21483. doi:Doi 10.1073/Pnas.0912138106-   Kose A R, Koser H (2012) Ferrofluid mediated nanocytometry. Lab on a    Chip 12 (1):190-196. doi:Doi 10.1039/C11c20864k-   Krebs Melissa D, Erb Randall M, Yellen Benjamin B, Samanta B, Bajaj    A, Rotello Vincent M, Alsberg E (2009) Formation of ordered cellular    structures in suspension via label-free negative magnetophoresis.    Nano Lett 9 (5):1812-1817.-   Laurell T, Petersson F, Nilsson A (2007) Chip integrated strategies    for acoustic separation and manipulation of cells and particles.    Chem Soc Rev 36 (3):492-506. doi:Doi 10.1039/B601326k-   Lee H, Purdon A M, Chu V, Westervelt R M (2004) Controlled assembly    of magnetic nanoparticles from magnetotactic bacteria using    microelectromagnets arrays. Nano letters 4 (5):995-998. doi:Doi    10.1021/N1049562x-   Lenshof A, Laurell T (2010) Continuous separation of cells and    particles in microfluidic systems. Chem Soc Rev 39 (3):1203-1217.    doi:Doi 10.1039/B915999c-   Liang L T, Zhu J J, Xuan X C (2011) Three-dimensional diamagnetic    particle deflection in ferrofluid microchannel flows.    Biomicrofluidics 5 (3):034110. doi:Artn 034110 Doi 10.1063/1.3618737-   Liu C X, Stakenborg T, Peeters S, Lagae L (2009) Cell manipulation    with magnetic particles toward microfluidic cytometry. J Appl Phys    105 (10):102014. doi:Artn 102014 Doi 10.1063/1.3116091-   Liu R H, Yang J N, Lenigk R, Bonanno J, Grodzinski P (2004)    Self-contained, fully integrated biochip for sample preparation,    polymerase chain reaction amplification, and DNA microarray    detection. Analytical Chemistry 76 (7):1824-1831. doi:Doi    10.1021/Ac0353029-   Love L J, Jansen J F, McKnight T E, Roh Y, Phelps T J (2004) A    magnetocaloric pump for microfluidic applications. Ieee T Nanobiosci    3 (2):101-110. doi:Doi 10.1109/Tnb.2004.828265-   Mao L, Koser H Overcoming the Diffusion Barrier: Ultra-Fast    Micro-Scale Mixing Via Ferrofluids. In: 14th International    Conference on Solid-State Sensors, Actuators and Microsystems, Lyon,    France, 2007. pp 1829-1832.-   Mao L D, Elborai S, He X W, Zahn M, Koser H (2011) Direct    observation of closed-loop ferrohydrodynamic pumping under traveling    magnetic fields. Phys Rev B 84 (10):104431. doi:Artn 104431 Doi    10.1103/Physrevb.84.104431-   Mao L D, Koser H (2006) Towards ferrofluidics for mu-TAS and lab    on-a-chip applications. Nanotechnology 17 (4):S34-S47. doi:Doi    10.1088/0957-4484/17/4/007-   Mihajlovic G, Aledealat K, Xiong P, Von Molnar S, Field M, Sullivan    G J (2007) Magnetic characterization of a single superparamagnetic    bead by phase-sensitive micro-Hall magnetometry. Applied Physics    Letters 91 (17):172518. doi:Artn 172518 Doi 10.1063/1.2802732-   Miller M M, Sheehan P E, Edelstein R L, Tamanaha C R, Zhong L,    Bounnak S, Whitman L J, Colton R J (2001) A DNA array sensor    utilizing magnetic microbeads and magnetoelectronic detection. J    Magn Magn Mater 225 (1-2):138-144.-   Miltenyi S, Muller W, Weichel W, Radbruch A (1990) High-Gradient    Magnetic Cell-Separation with Macs. Cytometry 11 (2):231-238.-   Mirica K A, Shevkoplyas S S, Phillips S T, Gupta M, Whitesides G    M (2009) Measuring Densities of Solids and Liquids Using Magnetic    Levitation: Fundamentals. Journal of the American Chemical Society    131 (29):10049-10058. doi:Doi 10.1021/Ja900920s-   Moriarty A T, Clayton A C, Zaleski S, Henry M R, Schwartz M R,    Eversole G M, Tench W D, Fatheree L A, Souers R J, Wilbur D C (2009)    Unsatisfactory reporting rates: 2006 practices of participants in    the college of american pathologists interlaboratory comparison    program in gynecologic cytology. Arch Pathol Lab Med 133    (12):1912-1916. doi:2008-0793-CPR1 [pii]    10.1043/1543-2165-133.12.1912-   Nagrath S, Sequist L V, Maheswaran S, Bell D W, Irimia D, Ulkus L,    Smith M R, Kwak E L, Digumarthy S, Muzikansky A, Ryan P, Balis U J,    Tompkins R G, Haber D A, Toner M (2007) Isolation of rare    circulating tumour cells in cancer patients by microchip technology.    Nature 450 (7173):1235-1239. doi:nature06385 [pii]    10.1038/nature06385-   Nguyen N T (2012) Micro-magnetofluidics: interactions between    magnetism and fluid flow on the microscale. Microfluidics and    Nanofluidics 12 (1-4):1-16. doi:Doi 10.1007/S10404-011-0903-5-   Nguyen N T, Ng K M, Huang X Y (2006) Manipulation of ferrofluid    droplets using planar coils. Applied Physics Letters 89 (5):052509.    doi:Artn 052509 Doi 10.1063/1.2335403-   Odenbach S, Editor (2002) Ferrofluids: Magnetically Controllable    Fluids and Their Applications. Springer, London.-   Pamme N (2006) Magnetism and microfluidics. Lab Chip 6 (1):24-38.    doi:Doi 10.1039/B513005k-   Pamme N (2007) Continuous flow separations in microfluidic devices.    Lab Chip 7 (12):1644-1659. doi:Doi 10.1039/B712784g-   Pankhurst Q A, Connolly J, Jones S K, Dobson J (2003) Applications    of magnetic nanoparticles in biomedicine. Journal of Physics D:    Applied Physics (13):R167-R181.-   Peyman S A, Iwan E Y, Margarson O, Iles A, Pamme N (2009)    Diamagnetic repulsion-A versatile tool for label-free particle    handling in microfluidic devices. J Chromatogr A 1216 (52):    9055-9062. doi:Doi 10.10164 Chroma.2009.06.039-   Rife J C, Miller M M, Sheehan P E, Tamanaha C R, Tondra M, Whitman L    J (2003) Design and performance of GMR sensors for the detection of    magnetic microbeads in biosensors. Sensors and Actuators a-Physical    107 (3):209-218. doi:Doi 10.1016/S0924-4247(03)00380-7-   Rodriguez-Villarreal A I, Tarn M D, Madden L A, Lutz J B, Greenman    J, Samitier J, Pamme N (2011) Flow focussing of particles and cells    based on their intrinsic properties using a simple diamagnetic    repulsion setup. Lab on a Chip 11 (7):1240-1248. doi:Doi    10.1039/C01c00464b-   Rosensweig R E (1985) Ferrohydrodynamics. Cambridge University    Press, Cambridge.-   Shen F, Hwang H, Hahn Y K, Park J-K (2012) Label-Free Cell    Separation Using a Tunable Magnetophoretic Repulsion Force.    Analytical Chemistry. doi:10.1021/ac201505j-   Shevkoplyas S S, Siegel A C, Westervelt R M, Prentiss M G,    Whitesides G M (2007) The force acting on a superparamagnetic bead    due to an applied magnetic field. Lab Chip 7 (10):1294-1302. doi:Doi    10.1039/B705045c-   Shi J J, Huang H, Stratton Z, Huang Y P, Huang T J (2009) Continuous    particle separation in a microfluidic channel via standing surface    acoustic waves (SSAW). Lab on a Chip 9 (23):3354-3359. doi:Doi    10.1039/B915113c-   Sun Y, Kwok Y C, Nguyen N T (2007) A circular ferrofluid driven    microchip for rapid polymerase chain reaction. Lab on a Chip 7    (8):1012-1017. doi:Doi 10.1039/B700575j-   Sun Y, Nguyen N T, Kwok Y C (2008) High-throughput polymerase chain    reaction in parallel circular loops using magnetic actuation.    Analytical Chemistry 80 (15):6127-6130. doi:Doi 10.1021/Ac800787g-   Toner M, Irimia D (2005) Blood-on-a-chip. Annu Rev Biomed Eng    7:77-103. doi:Doi 10.1146/Annurev.Bioeng.7.011205.135108-   Tsutsui H, Ho C M (2009) Cell separation by non-inertial force    fields in microfluidic systems. Mechanics Research Communications 36    (1):92-103. doi:Doi 10.1016/J.Mechrescom.2008.08.006-   Voldman J (2006) Electrical forces for microscale cell manipulation.    Annu Rev Biomed Eng 8:425-454. doi:Doi    10.1146/Annurev.Bioeng.8.061505.095739-   Wang Z C, Zhe J A (2011) Recent advances in particle and droplet    manipulation for lab-on-a-chip devices based on surface acoustic    waves. Lab on a Chip 11 (7):1280-1285. doi:Doi 10.1039/C01c00527d-   Winkleman A, Perez-Castillejos R, Gudiksen K L, Phillips S T,    Prentiss M, Whitesides G M (2007) Density-based diamagnetic    separation: Devices for detecting binding events and for collecting    unlabeled diamagnetic particles in paramagnetic solutions.    Analytical Chemistry 79 (17):6542-6550. doi:Doi 10.1021/Ac070500b-   Yamada M, Nakashima M, Seki M (2004) Pinched flow fractionation:    Continuous size separation of particles utilizing a laminar flow    profile in a pinched microchannel. Analytical Chemistry 76    (18):5465-5471. doi:Doi 10.1021/Ac049863r-   Yellen B B, Hovorka O, Friedman G (2005) Arranging matter by    magnetic nanoparticle assemblers. P Natl Acad Sci USA 102    (25):8860-8864. doi:Doi 10.1073/Pnas.0500409102-   Yu M, Stott S, Toner M, Maheswaran S, Haber D A (2011) Circulating    tumor cells: approaches to isolation and characterization. J Cell    Biol 192 (3):373-382. doi:jcb.201010021 [pii] 10.1083/jcb.201010021-   Yung C W, Fiering J, Mueller A J, Ingber D E (2009)    Micromagnetic-microfluidic blood cleansing device. Lab Chip 9    (9):1171-1177. doi:Doi 10.1039/B816986a-   Zborowski M, Ostera G R, Moore L R, Milliron S, Chalmers J J,    Schechter A N (2003) Red blood cell magnetophoresis. Biophys J 84    (4):2638-2645.-   Zhang K, Liang Q L, Ai X N, Hu P, Wang Y M, Luo G A (2011a)    Comprehensive Two-Dimensional Manipulations of Picoliter    Microfluidic Droplets Sampled from Nanoliter Samples. Analytical    Chemistry 83 (20):8029-8034. doi:Doi 10.1021/Ac2017458-   Zhang K, Liang Q L, Ai X N, Hu P, Wang Y M, Luo G A (2011b)    On-demand microfluidic droplet manipulation using hydrophobic    ferrofluid as a continuous-phase. Lab on a Chip 11 (7):1271-1275.    doi:Doi 10.1039/C01c00484g-   Zhu J J, Liang L T, Xuan X C (2012) On-chip manipulation of    nonmagnetic particles in paramagnetic solutions using embedded    permanent magnets. Microfluidics and Nanofluidics 12 (1-4):65-73.    doi:Doi 10.1007/S10404-011-0849-7-   Zhu T T, Cheng R, Mao L D (2011a) Focusing microparticles in a    microfluidic channel with ferrofluids. Microfluidics and    Nanofluidics 11 (6):695-701. doi:Doi 10.1007/S10404-011-0835-0-   Zhu T T, Lichlyter D J, Haidekker M A, Mao L D (2011b) Analytical    model of microfluidic transport of non-magnetic particles in    ferrofluids under the influence of a permanent magnet. Microfluidics    and Nanofluidics 10 (6):1233-1245. doi:Doi 10.1007/S10404-010-0754-5-   Zhu T T, Marrero F, Mao L D (2010) Continuous separation of    non-magnetic particles inside ferrofluids. Microfluidics and    Nanofluidics 9 (4-5):1003-1009

Example 2

Currently, we are developing a type of water based ferrofluids that canfacilitate cervical cancer cells sorting. To maintain the nonmagneticproperties of cancer cells, cellular uptake of magnetic nanoparticlesshould be minimized. Interaction between cells and magneticnanoparticles were known to be caused by endocytosis and physicalattraction (Verma and Stellacci 2010), which are dictated by the surfaceproperties. Polyethyene glycol and phosphorylcholine based copolymer waschosen as the surfactant to stabilize magnetite (Fe₃O₄) nanoparticlesfor their excellent biocompatibility (Yuan, Armes et al. 2006;Józefczak, Hornowski et al. 2009). Biomemetic phospholipid polar groupwere also proven to inhibit non-selective cellular uptake ofnanoparticles (Ishihara and Takai 2009). Copolymer structure providesmore flexibility with anchoring group and functional group, however,multiple groups can also easily interact with several nanoparticlesleading to flocculation (Boyer, Whittaker et al. 2010). Once thecolloidal stable ferrofluids are developed, cancer cells viability andcellular uptake of nanoparticles will be measured (Samanta, Yan et al.2008). Positive results will enable the application of ferrofluidscombing microfluidic platform as the cancer cells sorter.

-   Boyer, C., M. R. Whittaker, et al. (2010). “The design and utility    of polymer-stabilized iron-oxide nanoparticles for nanomedicine    applications.” NPG Asia Mater 2: 23-30.-   Ishihara, K. and M. Takai (2009). “Bioinspired interface for    nanobiodevices based on phospholipid polymer chemistry.” Journal of    The Royal Society Interface 6(Suppl 3): S279-S291.-   Józefczak, A., T. Hornowski, et al. (2009). “Effect of poly    (ethylene glycol) coating on the magnetic and thermal properties of    biocompatible magnetic liquids.” Journal of Magnetism and Magnetic    Materials 321(10): 1505-1508.-   Samanta, B., H. Yan, et al. (2008). “Protein-passivated Fe3O4    nanoparticles: low toxicity and rapid heating for thermal therapy.”    Journal of Materials Chemistry 18(11): 1204-1208. Verma, A. and F.    Stellacci (2010). “Effect of Surface Properties on Nanoparticle—Cell    Interactions.” Small 6(1): 12-21.-   Yuan, J. J., S. P. Armes, et al. (2006). “Synthesis of Biocompatible    Poly[2-(methacryloyloxy)ethylphosphorylcholine]-Coated Magnetite    Nanoparticles.” Langmuir 22(26): 10989-10993.

It should be noted that ratios, concentrations, amounts, and othernumerical data may be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a concentration range of “about0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt % to about 5 wt %, butalso include individual concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. In an embodiment, the term “about” can includetraditional rounding according to the measurement technique and the typeof numerical value. In addition, the phrase “about ‘x’ to ‘y’” includes“about ‘x’ to about ‘y’”.

Many variations and modifications may be made to the above-describedembodiments. All such modifications and variations are intended to beincluded herein within the scope of this disclosure and protected by thefollowing claims.

We claim:
 1. A kit for separating two or more types of particles, thekit comprising a device and a plurality of magnetic particles that arebiocompatible with mammalian cells, wherein the device comprises: (1) atleast one first fluid inlet in fluidic communication with a flowchannel, wherein the flow channel is configured to receive the two ormore types of particles and the plurality of magnetic particles via theat least one fluid inlet; (2) a magnet configured to direct anon-uniform magnetic force onto the flow channel whereby the two or moretypes of particles and the magnetic particles are exposed to thenon-uniform magnetic force when introduced in the flow channel such thatthe two or more types of particles experience a magnetic buoyancy force;and (3) a plurality of outlets, wherein the magnet is configured so thatthe non-uniform magnetic force causes the two or more types of particlesto be separated and flow into different outlets based on the magneticbuoyancy force and volume of the two or more types of particles; whereinthe device is configured to separate the two or more types of particleswhen at least one of the two or more types of particles are cells. 2.The kit of claim 1, wherein the magnetic particles comprise iron oxideparticles, cobalt particles, cobalt ferrite particles, iron particles,and FePt particles, or a combination thereof.
 3. The kit of claim 1,wherein the magnetic particles are nano-size magnetic particles coveredby a surfactant.
 4. The kit of claim 3, wherein the nano-size magneticparticles have a diameter of 5 nm to 10 nm.
 5. The kit of claim 1,wherein, the magnetic particles are configured to form a magnetic fluidupon introduction to a carrier medium.
 6. The kit of claim 5, whereinthe carrier medium comprises water, hydrocarbon oil, kerosene, or acombination there.
 7. The kit of claim 1, further comprising a magneticfluid, wherein the magnetic fluid comprises the plurality of magneticparticles.
 8. The kit of claim 7, wherein the magnetic particles arenano-size magnetic particles covered by a surfactant, and the magneticfluid is a colloidal mixture of the nano-size magnetic particles in acarrier medium.
 9. The kit of claim 8, wherein the nano-size magneticparticles have a diameter of 5 nm to 10 nm.
 21. The kit of claim 19,wherein the carrier medium comprises water, hydrocarbon oil, kerosene,or a combination there.
 23. The kit of claim 1, wherein the device isconfigured to separate the two or more types of particles when at leasttwo of the two or more types of particles are cells.
 24. The kit ofclaim 1, wherein the magnet is disposed on one side of the device togenerate the magnetic force.
 25. The kit of claim 1, wherein the magnetcomprises a permanent magnet.
 28. The kit of claim 1, wherein the deviceis configured to be operated under a continuous flow.
 29. The kit ofclaim 1, wherein the plurality of outlets includes two outlets forseparating particles having two different volumes.
 30. The kit of claim22, wherein the device is configured to separate the cells label-free.31. The kit of claim 22, wherein the device is configured to separatethe cells without the cells being conjugated to magnetic beads.